1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) receiver receiving an OFDM modulated signal, and more particularly, to a method and apparatus for a OFDM receiver's carrier frequency offset (CFO) synchronization.
2. Description of the Related Art
Digital Television Terrestrial Broadcasting (DTTB) services have been available in North America and Europe since November 1998. Tsinghua University suggested a new standardization draft for China-oriented terrestrial digital television (DTV-T). This draft relates to a broadcasting standard called terrestrial digital multimedia/television broadcasting (DMB-T). DMB-T uses a new modulation scheme called time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM).
The Standardization Administration of China, established a standard for Terrestrial Digital Broadcasting, number GB20600-2006 entitled ‘Framing structure, Channel coding and modulation for digital television terrestrial broadcasting system’. The new official standard issued in August 2006, more generally called DMB-T/H (Digital Multimedia Broadcasting Terrestrial/Handheld) DMB-T/H is a result of work at both the Tsinghua University in Beijing and Jiaotong University in Shanghai and is thought to be more a co-existence of the two separate standards developed by these universities rather than an all embracing single standard integrating both approaches. Tsinghua's system TDS-OFDM (Time Domain Synchronous OFDM) uses multicarriers just like the DVB-T and Japanese ISDB-T whilst Jiatongs ADTB-T (Advanced Digital Television Broadcast Terrestrial) is a single carrier vestigial sideband system based on the US 8-VSB standard.
In DMB-T, inverse discrete Fourier transform (IDFT) is applied to data modulated and transmitted by a TDS-OFDM transmitter as in cyclic prefix orthogonal frequency division multiplexing (CP-OFDM). An innovation of the DMB-T standard that can improve the performance of the system is the design of the PN sequence frame header and symbol guard interval insertion that can achieve rapid and efficient channel estimation and equalization, A pseudo-noise (PN) (instead of a cyclic prefix) is inserted into a guard interval and used as a training signal. The PN sequence can also be used as a time domain equalizer training sequence.
The combination of a guard interval and a training signal, can reduce overhead when a broadcast signal is transmitted, increase channel use efficiency, and improving performance of a synchronizer and a channel estimator included in a DMB-T broadcast signal receiver.
FIG. 1 illustrates a structure of a time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) signal frame 100.
The TDS-OFDM frame 100 includes a frame head (also known as frame sync) and a frame body. The baseband symbol rates for both frame sync (frame head) and frame body are the same, and are defined as 7.56 MSPS.
The frame body is an inverse discrete Fourier transform (IDFT) block on which data to be transmitted is carried, and in general, the IDFT block includes 3,780 pieces (“symbols”) of stream data. In the time domain, samples in each block correspond to the 3780 sub-carriers in the frequency domain of the block. The block in its time domain has 3780 samples of the inverse discrete Fourier transform (IDFT) of the 3780 sub-carriers in its frequency domain. There are 36 symbols of system information and 3744 symbols of data in one frame body. Thus, an IDFT block size Nc is 3,780. Because there are 3780 carriers and the carrier spacing is 2 kHz, so the bandwidth of multi-carrier mode is 7.56 MHz.
The frame head can also be called ‘frame sync’ or ‘frame header’. The size of the frame head depends on a guard interval mode. In general the guard interval mode is 1/9 or ¼. When the guard interval mode is 1/9, the frame head size Lpn is 420 and the time interval of the header is 55.6 μs, and when the guard interval mode is ¼, the frame head size Lpn is 945 and the time interval of the header is 125 μs. When the guard interval mode is 1/9, the frame head includes 420 pieces of data including 255 PN sequences, a preamble before the PN sequences, and a postamble after the PN sequences. The pre-amble and post-amble are cyclical extensions of the PN sequences. In other words, 420 pieces of data (that is 1/9 the amount of the 3,780 pieces of data in the IDFT block) are used for the frame head. For all signal structure modes, the frame body includes 3780 symbols and the time interval of frame body is 500 μs. Thus, a single OFDM frame includes a frame head including 420 pieces of data and a frame body including 3,780 pieces of data and so the time interval of a signal frame is 555.6 μs, or 625 μs respectively.
The frame head includes pseudo-noise (PN) sequences, wherein PN used in the frame head can use sequences whose order is 8 (m=8). A PN sequence is defined as an 8th order m-sequence and is implemented by a Fibonacci Type linear feedback shift register (LFSR). Its characteristic polynomial may be defined as: P (x)=x8+x6+x5+x+1. When order m=8, 255 different sequences can be generated, and the sequences can be extended using a preamble and a postamble to be used in a guard interval.
The preamble and the postamble are repeated intervals of PN sequences for cyclic extension of the PN sequences. For example, cyclic extension is performed by adding the first 82 PN sequences of 255 PN sequences in a frame head to the end of the 255 PN sequences as a postamble and adding the last 83 PN sequences of the 255 PN sequences to the first of the 255 PN sequences as a preamble. Thus, the total size of frame head is 83+255+82=420 (Lpn=Lpre+Lm+Lpost).
The structure of the data frame 100 may vary according to a guard interval, and the number of pieces of data in each frame may be different.
Such a data frame is disclosed in Korean Patent Publication No. 2007-0024298.
OFDM systems provide orthogonal sub-carriers that guarantee exact reconstruction of the original data. To obtain orthogonality between subchannels in OFDM systems, one of the assumptions which are made, is exact knowledge of the carrier frequency at the receiver. However, OFDM systems are also susceptible to errors relating to carrier frequency offset (CFO). CFO generally arises when the demodulation carrier frequency does not exactly match the modulating carrier frequency. This may result from, for example, Doppler effect or mismatched crystal frequency at the transmitter and receiver. CFO between transmitter and receiver essentially destroys the orthogonality of the OFDM symbol and can cause inter-carrier interference (ICI) and inter-symbol interference (ISI). With CFO between transmitter and receiver some of the signal power will be transferred into interference power, i.e. noise, reducing the system performance. Thus Carrier Frequency Offset CFO is a major contributor to the inter-carrier interference (ICI) in OFDM systems. In OFDM systems, carrier frequency offset (CFO) must be estimated and compensated (synchronizing, tracking) at the receiver to maintain orthogonality. In OFDM systems it is desired to synchronize the carrier frequency at the receiver with the carrier of the transmitter. In TDS-OFDM, fast synchronization acquisition and channel estimation is performed using above-described PN sequence code that is a time domain synchronization signal.
In the prior art, a correlation operation is used as a method for the fast synchronization acquisition and channel estimation. The correlation operation is disclosed in a reference published by Z. W. Zheng, Z. X. Yang, C. Y. Pan, and Y. S. Zhu, titled “Novel Synchronization for TDS-OFDM-based Digital Television Terrestrial Broadcast Systems”, IEEE Trans. Broadcast., vol. 50, no. 2, pp. 148-153, June 2004. Thus, a carrier frequency offset (CFO) tracking range obtained by using the correlation operation between a received signal r(n) and a sequence PN(n) is ±NcΔf/2Lm, wherein Δf denotes tone spacing, and Nc and Lm are illustrated in FIG. 1. For example, when Nc=3,780, Δf=2 KHz, and Lm=255, a tracking range cannot be over ±15 KHz, and the tracking range is too narrow to be used in practice.
In the prior art, a double correlation operation is used as another method for the fast synchronization acquisition and channel estimation. The double correlation operation is disclosed in a reference published by F. Tufvesson, O. Edfors, and M. Faulkner, titled “Time and frequency synchronization for OFDM using PN-sequence preambles”, in Proc. VTC' 99, vol. 4, pp. 2203-2207, September 1999.
By using the double correlation operation disclosed in the reference, the CFO tracking range can be widened, however, the CFO estimation error increases.
Thus, by considering all cases in the prior art, a CFO tracking range cannot be reliably used in practice when the CFO tracking range is too narrow, and a CFO estimation error increases when the CFO tracking range is too wide.
Thus, an apparatus and method for estimating a CFO of an OFDM receiver and performing CFO synchronization are required.